Help needed with multiple selections from a created list - vba solution??

HI EXCEL COMMUNITY! I would like to be able to select multiple items from a list. In the attached file, I have my list, cells A3-A6. In cell C2 I have the option to select from a list. I would like cell C2 to show like cell F3. I would also like it to be available to the entire workbook. I have this programed so far in the VBA of the Workbook for columns 3 & 4, see below. However, the output isn't giving me the results I desire. Am I writing the VBA code incorrectly?

Private Sub Worksheet_Change(ByVal Target As Range)
'Code by Sumit Bansal from https://trumpexcel.com
' To allow multiple selections in a Drop Down List in Excel (without repetition)
Dim Oldvalue As String
Dim Newvalue As String
Application.EnableEvents = True
On Error GoTo Exitsub
If Target.Column = 3 Or Target.Column = 4 Then
  If Target.SpecialCells(xlCellTypeAllValidation) Is Nothing Then
    GoTo Exitsub
  Else: If Target.Value = "" Then GoTo Exitsub Else
    Application.EnableEvents = False
    Newvalue = Target.Value
    Application.Undo
    Oldvalue = Target.Value
      If Oldvalue = "" Then
        Target.Value = Newvalue
      Else
        If InStr(1, Oldvalue, Newvalue) = 0 Then
            Target.Value = Oldvalue & ", " & Newvalue
      Else:
        Target.Value = Oldvalue
      End If
    End If
  End If
End If
Application.EnableEvents = True
Exitsub:
Application.EnableEvents = True
End Sub